sizeof() function in C [duplicate]

Answer 1

There are lots of errors, here.

"sizeof(a) gives me output: 11 (length of the string)"

The length of the string is 10, not 11. sizeof(a) gives you the length of the array.

"why is it so, why isn't the output sizeof(a)=4 since when I try to print a it gives me an address value"

Here are two methods of "printing a" which do not "give you an address value":

puts(a);

and:

printf("%s\n", a);

so your logic is flawed, and this is the source of your confusion. "Printing a" only "gives you an address value" when you explicitly or implicitly elect to do so.

sizeof(a) gives 11 in this case because the C language defines the sizeof operator to give you the size of an array when an array is the operand. This, I'd argue, is the most natural behavior people would expect, so presumably that's why it is defined as such.

"and hence an integer."

In any case, an address is an address, not an integer. At best you could argue that it ought to give you the size of a pointer, but certainly not the size of an integer.

Answer 2

Whenever you refer to the name of the array in your program, It normally decays to a pointer to the first element of the array. One of the exception to this rule is the sizeof operator. So when you consider the following code.

int main()
{
   char a[] = "Visual C++";
   printf("sizeof(a)=%d\n",sizeof(a)); /* Here sizeof(a) indicates sizeof array */
   printf("a=%p",a); /* Here the array name, passed as an argument to printf decays into a pointer of type (char *) */
   return 0;
}

Answer 3

When sizeof is applied to the name of a static array (not an array allocated through malloc), the result is the size in bytes of the whole array. This is one of the few exceptions to the rule that the name of an array is converted to a pointer to the first element of the array, and is possible just because the actual array size is fixed and known at compile time, when sizeof operator is evaluated.

Answer 4

In the declaration char a[] = "Visual C++", a is an array of 11 char. So its size is 11 bytes.

In the declaration char *b = "Visual C++", b is a pointer to char. So its size is four bytes (in the C implementation you are using).

In the expression printf("%s", a), a is also an array. However, it is automatically converted to a pointer to the first element of the array. So a pointer to char is passed to printf.

This conversion happens automatically unless an array is the argument of &, sizeof, or _Alignof or is a string literal used to initialize an array of char. Because it happens automatically, people tend to think of array names as pointers. However, they are not.

Incidentally, sizeof is an operator, not a function.